In 1984, paleoanthropologist Walker, together with Richard Leakey and Kamoya Kimeu, discovered the 1.5-million-year-old skeleton of a teenage male Homo erectus in Kenya. Dubbed the Nariokotome Boy after a nearby sand river, this hominid fossil reveals a tall, strong toolmaker, a cooperative, intensely social hunter who, though adapted to the tropics, was not fully human because, according to the authors, he did not possess language or think as we do. In an exciting first-person narrative coauthored with his paleoanthropologist wife, Walker uses the Nariokotome Boy and other finds to buttress his conjecture that our Homo erectus ancestors migrated out of Africa via the Middle East into Eurasia. In his analysis, Homo erectus, a ""missing link"" between apes and humans, experienced the prolongation of childhood typical of humans and mastered the human evolutionary trick of bearing big-brained babies whose brains continued to grow rapidly during the first year of life. Photos. (Mar.)